In the production of butter by machines, the fat and moisture content of the product must be closely regulated. Moisture gauges have been built previously to directly sense moisture in butter, but the gauges are far from accurate. The moisture usually has been sensed capacitively, using radio frequency sensors. However, the dielectric constant, which determines capacitance, varied not only with moisture, but also with density, a parameter not measured or detected by the prior moisture gauges. Density varies in butter because of different amounts of air introduced by the churning process. Although some butter machines employ vacuum sections to remove the air, the density variation still is large enough to render the dielectric constant measurement commercially useless as an indicator of moisture content.
Gamma-ray and X-ray gauges have been developed for determining material density by measuring the transmission of radiation through the material. The ratio (R) of radiation transmitted through the material (I.sub.1) to that detected in the absence of the material (I.sub.o) may be written: ##EQU1## where .mu. is the apparent mass absorption coefficient, .rho. is the unknown density and T is the thickness of the material. In a closed pipe, completely filled with the material, I.sub.1 and I.sub.o can be measured with radiation detectors, and since the thickness T is known the density may be determined on a continuous basis.
The composition of simple, essentially binary, mixtures such as butter (water and fat) has been shown to be measurable in the prior art using backscattered X-ray or gamma-ray radiation. (See for example: U.S. Pat. Nos. 4,168,431 and 3,255,975.)
In another invention, owned by the assignee of this invention, E. Dahlin has shown how a feedback control system using butter density and moisture content control signals, can be used as important parameters in operating a butter machine. While a static density and moisture control signals could be generated, for example by laboratory measurement, dynamic signals are preferable. By "dynamic signal" is meant a continuous real time or instantaneous measurement, as the butter is produced.
The need for a dynamic composition measurement signal exists not only in butter production, but in many industrial process applications, where the composition of the end product is slightly variable and knowledge of the variations is important.
In the same previously mentioned invention by E. Dahlin, a need is shown for measurement of the mass flow of butter from a butter machine. While average mass may be determined by weighing the product produced over a period of time, it is better to have a continuous measurement represented by a dynamic signal. A mass flow determination from a butter machine requires a real time density measurement, together with a measure of the butter flow velocity at a known dimensional cross section.